Hitherto, alternating magnetic fields have been used for magnetic devices using electromagnetic force such as transformers, electric motors, and power generators. In general, an alternating magnetic field is formed using a coil in the center of which a magnetic core is placed. In order to improve performance of a magnetic device or to reduce the size thereof, it is important for such magnetic core to have improved magnetic characteristics.
Therefore, for instance, in order to allow a magnetic core to have improved capacity to be molded or a reduced size depending on the magnetic device part, a dust core is used as a magnetic core in some cases. In a method for producing a dust core, first, a magnetic powder comprising a powder for a dust core, which is obtained by coating magnetic powder particles of iron or the like with an insulating layer consisting of a polymer resin such as a silicone resin, is prepared or produced. Next, the magnetic powder is introduced into a molding die and subjected to compression molding (compaction) under certain pressure conditions. Thereafter, in order to reduce iron loss (hysteresis loss) and the like, the dust core subjected to compression molding is subjected to annealing. In the case of the thus obtained dust core, eddy-current loss can be reduced with an increase in specific resistance by forming an insulating film. In addition, since a high-density dust core is obtained, magnetic characteristics, such as those relating to the magnetic flux density, can be improved.
For example, as a method for producing a dust core, a method for producing a dust core comprising: producing a powder for a dust core by subjecting a magnetic powder mainly consisting of iron (Fe) and silicon (Si) to heat treatment in an oxygen atmosphere at a dew point of −30° C. to 65° C. so as to form an insulating film on each magnetic powder particle; subjecting a magnetic powder comprising the powder for a dust core to compression molding; and carrying out annealing treatment in a nitrogen atmosphere (i.e., in a non-oxygen atmosphere) has been suggested. (See, for example, Patent Document 1.)    Patent Document 1: JP Patent Publication (Kokai) No. 2005-146315 A